Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

[David Daney <ddaney@caviumnetworks.com>]

> > This is what I thought.
> >
> > My cpu (Cavium Octeon) does not have out of order reads, so my wmb() is 
> Can you have reads that are out of order wrt writes? Because the above
> does not have out of order reads. It just had a read that came before a
> write. The above code could look like:
>
> (hypothetical assembly language)
>
>         ld r2, TASK_UNINTERRUPTIBLE
>         st r2, (current->state)
>         wmb
>         ld r1, (x)
>         cmp r1, 0
>
> Is it possible for the CPU to do the load of r1 before storing r2? If
> so, then the bug still exists.
Indeed it is.  Lockless operations make my head hurt.

Thanks for clarifying.

David Daney



> -- Steve
>
>
> > in fact a full mb() from the point of view of the current CPU.  So I 
> > think I could weaken my bariers in set_current_state() and still get 
> > correct operation.  However as you say...